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DGS-3610 Series Configuration Guide Chapter 35 IP Multicast Routing Configuration
group address. The BSR message is sent hop-by-hop within the whole domain. The device
receives and saves these BSR message. If the DR receives the member relationship report
of some group from the direct connection host and there is no routing item of this group, the
DR will use one Hash algorithm to map the group address to one candidate RP that can
serve for this group. Then, DR sends the Join/Prune message to the RP direction
hop-by-hop in the multicast way. If the DR receives the multicast data packet from the direct
connection host and there is no the routing item of this group, the DR will use one Hash
algorithm to map the group address to one candidate RP that can serve for this group. Then
the DR encapsulates the multicast data into the registration message and sends it to the RP
in the unicast way.
The PIM-SM and the broadcast/prune model-based PIM-DM differ in the aspects that the
PIM-SM is based on the explicit join model, that is, the receiver sends the join message to
the RP, but the router only forwards the packets of that multicast group on the output
interface that has joined a multicast group. The PIM-SM uses the shared tree to forward
multicast packets. Each group provides one Rendezvous Point (RP), and the multicast
source sends the data to the RP along the shortest path, and then the RP sends the data to
various terminals along the shortest path. This is similar to the CBT, but the PIM-SM does
not use the concept of core. One major advantage of the PIM-SM is that it not only receives
the multicast information through the shared tree, but also provides the mechanism for
conversion from the shared tree to the SPT. Such conversion reduces the network delay and
the possible congestion on the RP, but it consumes enormous router resources, so it is
suitable for the case that there are only a few multicast data sources and network groups.
The PIM-SM uses the shared tree and SPT to distribute multicast frames. Suppose that
other devices do not want to receive these multicasts unless otherwise stated definitely.
When a host joins a group, the equipment connected to the host must notify the root by
using the PIM join message, namely, the RP. This join message is transferred one after
another through the router, creating a shared tree structure. Therefore, the RP records the
transfer path and also the registration information from the first-hop router (DR) of the
multicast source, and uses these two types of information to improve the shared tree. The
branch/leaf information is updated through periodical query messages. In using the shared
tree, the multicast source first sends multicast packets to the RP in order to ensure that all
the receivers can receive them. *.G represents a tree, * represents all sources, and G
represents a particular multicast address. The prune information is also used in the shared
tree, that is, a branch/leaf does not want to receive the multicast frames.
The PIMv2 BSR is a method for distributing group-to-RP messages to all devices. The
PIMv2 BSR eliminates the demand to set the RP for each device. The BSR uses the
hop-by-hop to broadcast BSR messages and distribute the mapping information. At first, the
BSR is selected from the device. The election method is similar to the election of the
root-bridge on the L2 bridge, by using a priority value. Each BSR checks the BSR messages
and only forwards those with the priority higher than or equal to its own (higher IP address).
The selected BSR sends the BSR messages to the all-PIM-routers multicast group
(224.0.0.13), where TTL is 1. After the neighbor PIMv2 router receives the messages, it
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